Acoustic ridge waveguide

ABSTRACT

An improved acoustic waveguide structure which comprises a ridge waveguide disposed in a horizontal direction extending outwardly from a substrate. The horizontal orientation of the waveguide which is in the form of a ledge results in particle motion of the ledge predominantly normal to the substrate plane with the largest motion at the free edge of the ledge and almost no motion in the substrate, thereby permitting a plurality of such structures to be densely packed on a common substrate.

BACKGROUND OF THE INVENTION

This invention relates to acoustic waveguides in general and moreparticularly to an improved type of waveguide which permits greaterpacking densities and other advantages.

Surface wave acoustic devices are gaining widespread use as filters,delay lines and the like. In particular, in frequency ranges between 10mhz and 1 ghz, devices which are compact and provide numerous advantagesover inductive-capacitive type filters and tuned electromagneticwaveguides are possible. This results directly from the fact thatacoustic waves travel at a much slower speed than electromagnetic wavesand thus, the size of a structure can be correspondingly smaller in theorder of 10⁵.

When used in filtering applications these devices generally comprise apiezoelectric substrate on which are deposited two spaced transducers.The most common type of transducer used is what is known as theinterdigital transducer wherein a plurality of fingers extend from thetransducer pads on each side of the substrate and have overlappingportions. Electric fields created between the overlapping fingers of thetransducer excite the piezoelectric material to generate the surfacewaves. Also used are what are known as grating mode transducers in whicha grating of fingers coacts with a ground plane in much the same manner.

These conventional type waveguides where the waveguide is in the form ofa plate, have waves therein which are referred to as Rayleigh modes.These conventional surface waves are subject to loss of energy due todiffraction which necessitates the use of acoustic beams that are manywave lengths wide. Such an arrangement offers certain disadvantages,particularly when it is desired to densely pack a plurality ofwaveguides.

To overcome some of these problems, waveguides which propagate in thelowest order anti-symmetric flexural mode, have been developed.Generally, the proposed waveguides have been vertically orientedstructures such as that illustrated by FIG. 1. Considerable theoreticalanalysis has been performed on the performance of such a waveguide. Itsdispersion as a function of guide cross sectional geometry has beencomputed as described in an article by P. E. Lagasse, I. M. Mason and E.A. Ash, entitled "Acoustic-Surface Waveguides--Analysis and Assessment,"IEEE Trans MTT 21 No. 4, 225-235 (April, 1973), and in another articleby R. Burridge and E. J. Sabina entitled "The Propagation ofElastic-Surface Waves Guided by Ridges," Proc. R. Soc. Lond. A. 330 pps.417-441 (1972). The deformation associated with the different modes thatthe guide supports have been studied in the Burridge et al. articleabove and also in an article by C. C. Tu and G. W. Farnell entitled"Flexural Mode of Ridge Guides for Elastic Surface Waves," Elec Lett. 8No. 3 pps. 68-69, (Feb. 10, 1972). In the last mentioned article, thefield pattern penetration into the substrate was also presented.

In waveguides of this nature, the most tightly confined mode has a ridgestructure as illustrated by FIG. 2A. The computer generated flexureillustrated by that figure shows that a substrate is virtuallyundisturbed by the presence of the mode in the guide. It is the propertyof the guide that makes it especially important since it leads to thepossibility of a high density of non-interacting acoustic channelsadjacent to each other on a common substrate. The graph of FIG. 2Cillustrates the typical dispersion of this mode, i.e., the lowest orderanti-symmetrical flexural mode is illustrated by the curve 11. The factthat the velocity of this mode can be very low is desirable in achievinglong time delays. However, the dispersion evident in the lower branch isnot desirable unless controllable.

The other category of mode guided by the ridge is the modified form ofthe Rayleigh wave. When a ridge is present on a half space, the Rayleighmode becomes slowed down by the ridge and its amplitude falls off awayfrom the ridge. This is illustrated by the computer generated flexuralpattern of FIG. 2B. The inability of the guide to confine the energy ofthis mode makes it relatively uninteresting, particularly when it isnoted that the guide has added dispersion for this mode. Typicaldispersion for this mode is illustrated by the upper curve 13 of FIG.2C.

Prior experimental efforts in making waveguides of this nature havetaken basically two approaches. In one approach, thin verticallyoriented rectangular ridges, such as that of FIG. 1, of aluminum havebeen machined. By its very nature, this approach did not permit finegeometries to be achieved and consequently the guides were limited tolow frequency operation, i.e., less than 5 mhz. In addition, the natureof this type of approach results in a structure in which a high densityof adjacent guides cannot be fabricated. Furthermore, the excitation ofthe guides presents severe problems. Since aluminum is not apiezoelectric material, a separate transduction means must be attachedto the guide for the excitation of the modes. In the second approachwhich is disclosed in a paper by I. M. Mason, M. D. Motz and J. Chambersentitled "Wedge Waveguide Parametric Signal Processing," Proc.Ultrasonics Symposium, Boston, pps. 314-315, (October, 1972), one sideof a piezoelectric substrate is lapped to produce a wedge shapedstructure. The tip of the wedge then acts as the guiding ridge. Thistechnique permits easy excitation of the mode since the substrate ispiezoelectric. However, dispersion is difficult to control because ofthe craftsman-type construction method and a high density of guides isimpossible to achieve. The main disadvantages of the prior artapproaches are as follows:

1. Use of vertically oriented ridges does not lead to highly repeatablegeometries and a high density of waveguides at VHF frequencies, unlesssubstrates are used that admit to orientation dependent etching so thatguide walls can be defined precisely by crystal planes. Presently, thematerial capability in this regard exists only for silicon which is anon-piezoelectric material. In turn, the use of silicon may lead tosignificant technological difficulties in the excitation of the requiredmodes discussed below;

2. Even if highly precise repeatable guide geometries could be produced,the dispersion of the guide is not amenable to alteration, i.e., thereis no parameter available for modification that will permit thedispersion in certain range of wave lengths to be flattened;

3. The motion of the guide is essentially parallel to the plane of thesubstrate and thus, not along the deformation direction of currentlysputtered piezoelectric material.

Thus, it can be seen that although waveguides excited by tightlyconfined modes can offer distinct advantages, there is a need for animproved waveguide of this nature which does not suffer the above-noteddrawbacks.

SUMMARY OF THE INVENTION

The present invention provides such a waveguide structure which avoidsthe above-noted disadvantages and discloses various ways of employingsuch a structure. This is accomplished in a waveguide structure whichhas a horizontal ledge extending from its attachment point at asubstrate. With such a structure, the particle motion of the ledge ispredominantly normal to the substrate plane. edge this arrangement, themotion is largest at the free ledge of the ledge whereas at the attachedbase of the ledge the motion is very slight and decays rapidly to zerowithin the substrate material. Thus, the acoustic energy in the ledgepropagates along the ledge and is tightly confined to the ledge region.The disturbance extends into the substrate only a small fraction of thewavelength. Thus, tight packing of such waveguides is possible.

Various applications of the waveguide structure of the present inventionare also disclosed. Its use in resonators, delay lines and filters isdisclosed. Furthermore, the application of the ridge waveguide forgeneration non-linear interactions and an embodiment for such are alsodisclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical ridge waveguide such as those known in the priorart.

FIG. 2A is a computer generated flexure pattern for a ridge waveguidesuch as that of FIG. 1 guiding a tightly confined mode.

FIG. 2B is a similar illustration of a vertical ridge waveguide guidinga Rayleigh wave.

FIG. 2C is a graph illustrating the dispersion in the modes of FIGS. 2Aand 2B.

FIG. 3 is a perspective view illustrating one form of the horizontalridge waveguide of the present invention.

FIG. 4 is a perspective view of a ledge waveguide with a gold layerdeposited thereon.

FIG. 5 is a graph illustrating the effects of the gold layer on thedispersion in the waveguide of FIG. 4.

FIG. 6 is a view of the waveguide of FIG. 4 with a piezoelectric layerand a transducer deposited thereon.

FIG. 7 is a perspective view illustrating a horizontal ridge waveguideconfigured as an open circuit resonator.

FIG. 8 is a perspective view similar to FIG. 7, but showing a shortcircuited resonator configuration.

FIG. 9 is a perspective view similar to FIG. 7, but showing anelectronically closed loop resonator.

FIGS. 10A and 10B are respectively elevation and plan views of a closedloop resonator wherein the closed loop is obtained in a mechanicalmanner.

FIGS. 11A and 11B are perspective views illustrating a resonator havingperiodic reflectors on its ledge.

FIG. 12 is a perspective view illustrating the application of thehorizontal waveguide of the present invention in a delay line usingactive regeneration and coupling between ridges.

FIG. 13 is a perspective view of a delay line structure wherein couplingbetween ridges is done through a vertical ridge guide connecting loop.

FIG. 14 is a perspective view illustrating the use of the ridgewaveguide in a transverse filtering arrangement.

FIG. 15 is a perspective view illustrating the manner in whichnon-linear interaction may be obtained with a horizontal ridge guidesuch as that of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 3 illustrates the waveguide structure of the present invention andthe type of mode excited therein. Shown is a substrate 21 having a ledge23 extending therefrom. As illustrated, an acoustic mode is exicted inthe ledge causing it to flex up and down. The particle motion of theledge is predominantly normal to the substrate plane. The drawing alsoillustrates in schematic form, the relative degree of motion at variouspositions in the ledge. As shown, at the free end of the ledge themotion is largest whereas at the attached base of the ledge the motionis very slight, and in the substrate material, the motion decays to zerorapidly. Thus, acoustic energy in the ledge propagates along the ledgeand is tightly confined to the ledge region. Disturbances extend intothe substrate only a small fraction of a wavelength. Typical dimensionsof the ledge are on the order of 1/3 wavelength thick by 1 wavelength inlateral extent outwardly from the substrate.

The construction of the present invention also lends itself tocontrolled dispersion. Dispersion may be altered in a controlled mannerthrough the evaporation of metals onto the exposed broad wall of theguide. This is illustrated by the embodiment of FIG. 4 which shows asingle ledge of the type described in connection with FIG. 3. Asillustrated, the silicon ridge 37 is mass loaded with a layer 39 ofevaporated gold. The effect of such loading is illustrated by the curvesof FIG. 5. The top curve 41 is for an SiO₂ guide having a thickness wand a height 3w. The bottom curve 43 is for a ledge which is one-halfSiO₂ and one-half gold. The modification of the dispersion is evidentfrom the curve. In the illustrated case, the velocity is slowed by afactor of approximately one-half while the fractional change in velocityis not altered appreciably. Also the example of FIG. 5 is for a guide ofSiO₂ rather than silicon, but the results extend qualitatively to anyguide with acoustic properties quite dissimilar from those of gold.

The most important feature of the horizontal orientation is thepotential for excitation of the guide. With the horizontal arrangement,the broad side of the guide is exposed and is lying in the plane of thesubstrate. This allows grating mode transducers to be fabricateddirectly on the broad wall of the guide over a sputtered piezoelectricfilm. An arrangement of this nature is illustrated by FIG. 6.

A ledge 37 is formed in silicon after which the ledge is mass-loadedwith a layer 39 of gold as described above in connection with FIG. 4.The mass loading with gold not only slows the velocity of the modeleading to longer time delays but in addition, the gold serves as aground electrode under the piezoelectric film and also acts to orientthe film deposited on it, thus, ensuring high coupling. As illustrated,over the gold a piezoelectric layer 45 of ZnO is then deposited bysputtering, after which a grating mode transducer 47 is deposited overthe ZnO film 45. An opening is made through the film 45 for attachmentof a lead 49 to the ground plate 39 of gold. A suitable excitation means51 may then be used to cause the grating mode transducer 47 to exciteacoustic waves in the ledge 37.

An application of the waveguide of the present invention as an opencircuit resonator is illustrated by FIG. 7. In this embodiment, a ledge37 is formed, which ledge is discontinuous at each end. A pair oftransducers 61, 63 are deposited thereon in the manner described above.It should be noted that if a method of ledge formation such as thatdisclosed in copending application entitled "The Chemical Fabrication ofOverhanging Ledges and Reflection gratings for Surface Wave Devices"Donald F. Weirauch, Ser. No. 429,475, filed Dec. 28, 1973, is used, thestarting material is piezoelectric and transducers such as thetransducers 61 and 63 shown in FIG. 7 can be directly deposited onto theledge 37. In that case, the underside of the ledge can have a groundplane deposited thereon rather than using the gold and ZnO layersdiscussed above in connection with FIG. 6. The transducers 61 and 63will be coupled to a source and a load in conventional fashion. This isnot shown in order to maintain simplicity in FIG. 7. In the embodimentof FIG. 7, the acoustic mode is reflected from the discontinuity on theledge and passes back and forth beneath the transducers. There is asynchronous frequency for which the round trip propagation between thetwo ends of the guides gives a phase shift of 2nπ, at which transmissionbetween the ports becomes large, thus, achieving a filtering action.Thus, what has been shown is an open circuit resonator. Such resonatorscan be used as control elements to control the frequency of RFoscillators and may also be used as individual resonators in multi-poleband pass filters. Thus, a flexural ridge guided mode can be used as ameans for trapping energy in a very small space to form a resonator. Asnoted above, the sizes involved for typical frequencies are generallymuch smaller than those obtainable with conventional components.Although illustrated using a wedge shaped waveguide which can be formedas described in the previously referenced patent application, othertypes of cross-sectional geometries such as discussed in connection withFIGS. 3, 4 and 6 may also be used. Dispersion is not particularlyimportant in a resonator. Tight confinement, repeatability offabrication and low propagation loss are the most important factors. Inaddition, the key feature needed in a resonator is that of providingmeans for feeding the signal back on itself to form a high Q standingwave resonator. One form of achieving this is the open circuit resonatorof FIG. 7.

A second means of obtaining such operation is the short circuitresonator illustrated by FIG. 8. It should be noted at this point thatthe open circuit resonator and short circuit resonators of FIGS. 7 and 8respectively are the analogs of open circuit or short circuitterminations in electrical waveguides. If the material velocity vectoris defined as being equivalent to a voltage and the acoustic stresstensor as the analog of an electric current, then the simple structurein FIG. 7 is the analog of an electrical waveguide resonator where theends are open circuited; that is the acoustic stress vanishes at theends of the short waveguide section and the acoustic displacementobtains a maximum at these ends. The transducers shown are theequivalent of a pair of coupling loops in an electro-magnetic resonantwaveguide cavity. Such a pair of coupling transducers are useful in abandpass filter configuration with one transducer used for coupling intothe resonator and the second transducer used for coupling back out ofthe same resonator structure. In a case where only a one port device isdesired as in an RF oscillator, then only a single transducer isrequired.

The short circuited configuration of FIG. 8 differs from theconfiguration of FIG. 7 in that the opposite ends of the ledge of thewaveguide respectively merge into the body of the substrate so as to becontinuous therewith. Again, this is equivalent to a short circuitedelectro-magnetic waveguide and what has been said above in regard totransducers and couplings applies equally to this configuration. Eitherconfiguration may be used to obtain high Q resonant structures.

FIG. 9 illustrates an electronically closed loop resonator. Thestructure is essentially the same as that of FIG. 7 with the primarydifference being that a gain block 67 is used to couple the transducers61 and 63. The signal from a transducer 61 is amplified in a gain block67 such as an amplifier and then fed back to the transducer 63. Theamplification is used to overcome propagation and transducer losses. Theresonator shown on FIG. 9 is similar to the surface wave delay lineresonators for use in UHF oscillators which are described in copendingU.S. patent application Ser. No. 301,918 filed Oct. 30, 1972, now U.S.Pat. No. 3,868,595. In the present embodiment, it is desired to damp thesignals which leak past the transducers such that spurious resonances donot occur but rather that the resonance is precisely controlled by thespacing between the two transducers. A major advantage of this resonatorover previous structures is that the resonance can be more preciselycontrolled by a simple metallization pattern, i.e., by the spacing ofthe transducers, and that the Q of the resonator can be more easilycontrolled externally by varying the gain in the loop.

Another manner of obtaining a closed loop resonator is illustrated bythe embodiment of FIGS. 10A and 10B. A cylindrical ledge 71 is formed ina silicon wafer in a suitable manner. A piezoelectric layer 72 andnecessary loading and transducers 73, 74 are placed thereon in themanner described in connection with FIGS. 6 and 7. The transducers 73and 74 shown in FIGS. 10A and 10B have radially extending fingers. Inthis case, the wave guiding structure forms a closed loop in the form ofa circle. No reflection is required with the signal simply runningaround in a closed path.

FIGS. 11A and 11B illustrate another manner of obtaining resonance. Asillustrated in FIG. 11A, a ledge 37 similar to that of FIG. 7 is formedupon which is placed a single transducer 75. The underside of the ledge37 contains a plurality of indentations 77 which act as perturbations orreflectors. Although shown in detail in FIG. 11B as being formed bycut-outs or notches on the bottom side of the ledge 37, other forms ofperturbations may also be used. This construction forms a periodiccorrugation on the waveguide structure. As the acoustic waves encounterthese perturbations, they will be reflected with the resonance beingdetermined by the spacing of the perturbations. Another manner ofobtaining reflectors to accomplish the same result is through thedeposition on the ledge 37 of another material such as gold, zinc oxideor various other low loss acoustic materials and then the patterning ofthese materials, possibly during the same step as when forming thecoupling transducers, to define reflector strips selectively positionedon the ledge 37. Such patterned zinc oxide structures have been shown tobe very successful for similar resonators which operate using standardRayleigh waves.

As noted above, the major advantage of the ridge waveguide in a resonantstructure as compared to conventional structures using Rayleigh waves isprimarily in its compactness which permits a large number of resonantstructures to be built into a small area and also allows very compactbandpass filters and resonators for RF circuit applications to beconstructed. The latter can be constructed as an integrated circuitbandpass filter.

The waveguides of the present invention are also applicable for use asdelay lines and in particular lead to the ability to construct extremelylong delay lines which take up very little room. Delay lines constructedusing the waveguides of the present invention can be used as analogmemory for such things as single line or frame storage for a televisionpicture or as memory for use in correlation from one line to another ina television image. In addition, these waveguides have digitalapplications and are particularly useful in cases where extremely highspeed digital data is used. They may also be used in other well-knowndelay line applications. For example, these devices can be used as aclutter reference for a radar or as a transmitter frequency referencefor a radar system. The primary feature of the flexural acoustic guidedwave for use in long delay lines is that the wave is tightly confined toa very small guide permitting a large number of guides to be packagedvery closely together to obtain a very compact delay line and furtherthat the high confinement possible in such waveguides means that therewill be no cross talk between adjacent waveguides.

Since the waveguides cannot always be of a length sufficient to obtainthe required delay, means must be provided for changing the direction ofwave propagation in the guide on the surface. In a construction such asthat described above in connection with FIG. 3, it may be possible tobuild in changes in direction. However, difficulties arise when usingthe type of wedge structure disclosed in the aforesaid co-pending U.S.patent application Ser. No. 429,475, filed Dec. 28, 1973 previouslymentioned. Thus, in waveguides made according to such techniques,turning of corners in the waveguide structure is not practical.

FIG. 12 illustrates, however, one technique wherein such turning may beaccomplished. A plurality of waveguides 81a and 81b are shown. The inputis provided to the transducer 83a which excites the wave in thewaveguide 81a. The transducer 83b converts the acoustic energy back toelectrical energy which is then provided to a regeneration amplifier 85whose output is coupled to the transducer 83c on the waveguide 81b. Theacoustic wave generated by this transducer is then transmitted to thetransducer 83d where it is again converted to electrical energy and maybe provided as an output. Although only two passes through waveguidesare shown in FIG. 12, it will be evident that as many as required may beused with regeneration amplifiers 85 coupling each succeeding pair ofwaveguides. The amplifier 85 may be integrated directly onto thesubstrate. This arrangement offers a further advantage in that eachregeneration point coupling two waveguide structures may also be used asa tap, thus making information stored in the delay line accessible atmany points.

Another manner of turning a corner is illustrated by FIG. 13. Aillustrated, at the end where the waveguide 81a is coupled to thewaveguide 81b, the structure is modified so that it goes from being ahorizontal structure to a vertical structure 87. The vertical structure87 is then used to turn the corner whereupon it is then changed back toa horizontal structure. The end portion can be fabricated withomnidirectional etching.

Although both the wedge type waveguide structure which can be made asdescribed in the aforesaid copending application and the structuredisclosed in connection with FIGS. 3, 4, and 6 can both be used in manyapplications, the wedge waveguide has features which make it preferable.In particular, it has better dispersion qualities. In long delay linesminimizing dispersion is quite important since dispersion tends tospread the R.F pulses over a longer time length and tends to round offsharp rising and falling wave forms. For example, consider a delay linewith an nominal time delay τ at its center frequency and a fractionalchange in propagation velocity across its band width equal to ΔV/V.Further, let T₁ be the time width of the smallest pulse which is desiredto be transmitted faithfully through the delay line. The amount such aspulse will spread, designated T_(s) is as follows:

    t.sub.s ≃ τ Δ  V/V

if the condition is imposed that the amount of spread of the pulse beless than the width of the shortest pulse which must be passed, thefollowing equation holds:

    T.sub.s <  T.sub.1

combining these two equations the following is obtained;

    T.sub.1 > τ Δ  V/V

or

    Δ V/V <  T.sub.1 /τ

from this, the ratio of τ divided by T₁ can be recognized as the usefulnumber of bits of information which may be stored in the guide and maybe designed by N. From this, the dispersion across the band width of thedelay line must satisfy the following condition:

    Δ V/V <  1/N

for most cases of practical interest, the number N will range frombetween 100 and 100,000 thereby making the dispersion requirementsextremely stringent. As a result, the only flexural guided mode which isof practical use for long delay lines is that which occurs in a wedgeshaped waveguide. Such a waveguide must have a wedge that is taller thanan acoustic wavelength at the lowest frequency of interest in order toachieve acceptably low dispersion characteristics such as thosedescribed in the Lagasse et al. article noted above.

While the wedge waveguide is the only structure which is nearlynon-dispersive, the other geometries discussed above which do have highdispersion may be used in other types of applications such as in formingdispersive delay lines, which are used in radar systems and for pulsecompression filters.

The wedge waveguide structure may also be used to build a class offilters referred to as transversal filters. These are filters thatbasically comprise a delay line wherein the signal passes under manytaps which sample the signal at different time delay increments and thesamples are then all summed. A large majority of the surface wavefilters currently in use or under development are transversal filters inthat both the input transducers and output transducers used in thesefilters comprise basically a plurality of small taps on a surface delayline which are used either to generate or detect a small portion of thetotal wave. Transversal filters are an extremely flexible class offilters which are used for such diverse applications as bandpassfilters, pulse compression filters, phase coded correlators, frequencydiscriminators, and specialized filters which require unusual amplitudeand/or phase responses, possibly in several different bandpass regionsin a single filter. Transversal filters can be built using thenon-dispersive delay lines described above with a plurality of multipletaps placed on the delay line structure to coincide with the desiredimpulse response of the filter.

An example of such use is illustrated in FIG. 14. Here transducers 91and 93 are deposited on the wedge shaped waveguide structure 95. Thetransducers 91 and 93 each have an FM periodicity. Such an arrangementis useful as a pulse compression filter for a radar system, for example.In principal, all surface wave devices now in existence can beconstructed using the wedge or horizontal waveguide of the presentinvention and in particular, through the use of the wedge waveguide.

A further application of the flexural ridge guide is in its use forgeneration non-linear interactions. Non-linear acoustic correlators havebeen previously developed and disclosed for both surface wave devicesand flexural ridge guided wave devices. For example, such is disclosedin an article by I. M. Mason and M. D. Motz and J. Chambers entitled"Wedge Waveguide Parametric Signal Processing" Proc. UltrasonicsSymposium, Boston, 314-315, (Oct., 1972). In general, non-linearinteractions are not of particular interest for standard surface wavedevices because extremely high power levels must be used before asufficient power density is achieved to observe acoustic non-linearitiesthat are large enough to be useful. However, with a ridge guided mode,the acoustic energy is confined to less than one wavelength whereastypical surface wave devices have this energy spread over a distance onthe order of 100 wavelengths wide. Therefore, with the ridge waveguide,extremely high power densities may be easily achieved. This isparticularly true in the wedge-shaped guide because the strain at thetip of the wedge becomes very large. This permits the flexural ridgeguided mode to be used for non-linear interactions and in particular,for mixing applications where it is desired to obtain either the sum ordifference of two frequencies of interest.

Such an arrangement is shown in FIG. 15. Here, a long flexural modedelay line is used as a frequency controlling element for a feedbackoscillator. A flexural ridge waveguide 101 is provided and has depositedon its ends transducers 103 and 105. The spacing between the transducer103 and the transducer 105 is made much larger than the reciprocal ofthe band width of the transducers. Thus, there are many frequencies atwhich the oscillator can resonate. The particular frequency is chosen bya low Q external circuit. Thus, this could easily form the localoscillator for a receiver having equal channel spacings, for example,100 KHZ. An interdigital transducer 107 is deposited on the top of thestructure and is used to generate a Rayleigh wave in the direction ofarrow 109. The surface wave so generated will be incident obliquely onthe ridge in its center portion where the strain is large. The resultwill be such that the difference of the surface wave vector and theridge guided mode wave vector correspond to the periodicity of an outputtransducer III placed on the center of the delay line and which operatesat the desired IF frequency for the receiver application. The receivedsignal in this case is transduced into a normal Rayleigh surface waveand simultaneously filtered by the input transducer. The output couplingelectrode on the transducer III forms the IF filter for this particulardevice. With proper design, a device such as this can be used to providea receiver front end having a reasonably low noise figure and a widedynamic range. By installing a plurality of transducers 107 connected inparallel at appropriate angles, each corresponding to a differentfrequency, or one curved transducer as shown, a very simple tunablereceiver where the only tuning required is the external tuning of theoscillator loop can be provided. Furthermore, an additional feature ofthis type of non-linear mixing is that, since propagating waves areinvolved, there is a wave vector selection as well as a frequencyselection which must be satisfied and hence single-side band mixing isobtained, i.e., no images are present. A modification of the embodimentof FIG. 15 would be in the deposition of another broad band inputtransducer on the ridge guided delay line so that the input signal wouldbe coupled into the delay line in the form of a ridge guided waveinstead of a Rayleigh wave. The advantage of this configuration over theprevious one would be increased efficiency and a more compact device.Its disadvantage is a more complex fabrication for the ridge guidedstructure and a wider necessary operating band width for the ridgeguide.

Thus, an improved type of ridge waveguide and a plurality ofapplications therefor have been shown. Although specific embodimentshave been illustrated and described it will be obvious to those skilledin the art that various modifications may be made without departing fromthe spirit of the invention which is intended to be limited solely bythe appended claims.

What is claimed is:
 1. An acoustic waveguide structure comprising:asubstrate formed of a single crystal piezoelectric material and having atop surface lying in a horizontal plane; and a wedge-shaped horizontalledge formed of said single crystal piezoelectric material extendinglaterally outwardly from said substrate and having a top surfacecoplanar with the top surface of said substrate.
 2. An acousticwaveguide structure as set forth in claim 1, further including meansoperably associated with the top surface of said ledge for excitingacoustic waves therein.
 3. An acoustic waveguide structure comprising:asubstrate formed of a single crystal piezoelectric material and having atop surface lying in a horizontal plane; and a horizontal ledge formedof said single crystal piezoelectric material extending laterallyoutwardly from said substrate and having a top surface coplanar with thetop surface of said substrate, said ledge being of the order of 1/3wavelength thick and one wavelength in lateral extent outwardly fromsaid substrate.
 4. An acoustic waveguide structure comprising:asubstrate formed of single crystal silicon, said substrate having a topsurface lying in a horizontal plane; a wedge-shaped horizontal ledgeextending laterally outwardly from said substrate and having a topsurface coplanar with the top surface of said substrate; and meansoperably associated with the top surface of said ledge for excitingacoustic waves therein.
 5. An acoustic waveguide structure comprising:asubstrate formed of single crystal silicon, said substrate having a topsurface lying in a horizontal plane; a horizontal ledge formed of saidsingle crystal silicon extending laterally outwardly from said substrateand having a top surface coplanar with the top surface of saidsubstrate; a metal layer deposited over the top surfaces of said siliconledge and silicon substrate; and means operably associated with the topsurface of said ledge for exciting acoustic waves therein, said acousticwave-exciting means comprising a piezoelectric layer over said metallayer and a transducer deposited on said piezoelectric layer so as to bedisposed over said ledge.
 6. An acoustic waveguide structure as setforth in claim 5, wherein said metal layer is gold and saidpiezoelectric layer is zinc oxide.
 7. An acoustic waveguide structurecomprising:a substrate formed of single crystal silicon, said substratehaving a top surface lying in a horizontal plane; a horizontal ledgeextending laterally outwardly from said substrate and having a topsurface coplanar with the top surface of said substrate, said ledgebeing of the order of 1/3 wavelength thick and one wavelength in lateralextent outwardly from said substrate; and means operably associated withthe top surface of said ledge for exciting acoustic waves therein.
 8. Anacoustic waveguide structure comprising:a substrate made of a materialtaken from the group consisting of single crystal silicon and singlecrystal piezoelectric material; a ledge integral with said substrate andextending laterally outwardly with respect to a first surface of saidsubstrate; said ledge having a surface corresponding to and coplanarwith a second surface of said substrate which is perpendicular to saidfirst surface of said substrate; and said surface of said ledge beingcontiguous with said second surface of said substrate along the extentof said ledge.
 9. An acoustic waveguide structure comprising:a substrateformed of single crystal silicon; a ledge integral with said substrateand extending laterally outwardly with respect to a first surface ofsaid substrate; said ledge having a surface corresponding to andcoplanar with a second surface of said substrate which is perpendicularto said first surface of said substrate; said surface of said ledgebeing contiguous with said second surface of said substrate along theextent of said ledge; and means operably associated with said surface ofsaid ledge for exciting acoustic waves therein.